The use of communication cables which include a plurality of optical fibers is rapidly expanding. An optical fiber cable may comprise a plurality of glass fibers each of which is protected by at least one layer of a coating material. The optical fibers may be assembled into units in which the fibers are held together by binder ribbons or tubes to provide a core. Another optical fiber cable core includes a ribbon type optical fiber arrangement in which a plurality, such as twelve fibers for example, are arrayed together side by side. A plurality of these ribbons may be stacked to obtain a high fiber count cable. The core is enclosed by a plastic core tube and a plastic jacket. Ribbon type cable in which a relatively large number of optical fibers may be packaged appears to be ideally suited for fiber-to-the-customer use.
Whatever the structure of a cable, there must be provisions for splicing transmission media at an end of a given length of cable to corresponding transmission media at an adjacent end of another length of cable. In wire-like metallic conductor communication practice, it is conventional to use a splice closure, within which strength members of the cable ends may be anchored and all conductors spliced, wrapped and stored and protected environmentally. During the splicing of metallic conductors, it is customary to bend sharply the conductors, to provide access to other connections.
The physical nature of glass optical fibers forecloses the adoption of splicing techniques which are used with metallic conductors within such a splice closure. Because of their small size and relative fragility, special considerations must be given to the handling of optical fibers in closures. Transmission capabilities may be impaired if an optical fiber is bent beyond an allowable bending radius, the point at which light no longer is totally contained in the core of the fiber. Furthermore, fibers are brittle and their expected lives will be reduced if bent more than the minimum bending radius. Generally, the radius to which the optical fiber can be bent without affecting orderly transmission is substantially greater than that radius at which the optical fiber will break. Whereas glass and silica, the materials used to make optical fibers, are in some respects stronger than steel, optical fibers normally do not possess this potential strength because of microscopic surface fractures, which are vulnerable to stress and spread, causing the fiber to break easily.
It should be clear that, an optical fiber cable does not lend itself to the splicing practices of wire-like communication conductors. The individual glass fibers cannot just be twisted, tied, wrapped and moved into a splice closure, in anything like the manner of wire-like metallic conductor cables. These small diameter glass fibers cannot be crimped or bent at small angles, without breakage. Inasmuch as glass fibers have memory and tend to return to a straight-line orientation, placement in a splice closure becomes somewhat difficult. Moreover, the interconnection of optical fibers is a precision operation which in the past has tended to discourage some from performing splicing operations within a manhole, in a duct, or a pole-suspension elevation. And yet, to do otherwise becomes more expensive.
These problems are particularly acute in multifiber cables where individual optical fibers must be spliced in a manner which allows repairs and rearrangements to be made in the future. In addition, fiber slack normally must be provided adjacent to the splices. The need to store the slack further complicates the problem of providing a suitable optical fiber closure.
When splicing optical fibers by fusion or by mechanical means, it becomes necessary to provide enough slack fiber so that the fiber can be pulled out of the splice case for the preparation of fiber ends and the joining together. This requires at least about 0.5 meter of fiber from each cable to be stored in the splice closure when the closure is sealed, that is when the splicing has been completed. For a multifiber cable there must be a method of storing this slack, of protecting the splice and of keeping the fibers together in an orderly manner. The splices should be easily accessible to facilitate the rearrangement of the optical fibers and splices.
Optical fiber connecting arrangements must be protected from forces which could distort their shape or pull the fibers out of the arrangements. Although it is important that large forces are not applied to the connective arrangements, it also is important that they be secured in position. Any axial or torsional movement thereof could cause movement of the fibers which could cause attenuation of the optical signal being transmitted therealong.
Furthermore, there are a number of different kinds of splicing arrangements which are used commercially. Desirably, a closure should be capable of accommodating at least the more popular of these splicing arrangements. Also, because of the thrust toward fiber-to-the-customer architectures which may take place in the not too distant future, a closure must be able to accommodate a larger number of splices than has been customary in the art.
The above-enumerated problems must be overcome inasmuch as it is necessary to splice together the ends of optical fiber cables in field locations. A new closure is sought after to facilitate splicing in which suitable protection is afforded the optical fibers. Provisions must be included in the sought-after splice closure for storing several kinds of splicing arrangements.
As must be expected, fiber splice organizers and splice closures are available in the prior art. These prior art organizers and closures have suffered from a variety of shortcomings. Typically, they have been somewhat complex, difficult to use and difficult to access. Some of the prior art splice organizers have tended to place undue stresses on the optical fibers resulting in fiber damage. In addition, these prior art closures often have failed to provide simple to use, positive means for routing the optical fibers in an effective manner to the organizers and for storing slack.
For example, a splice closure with a central transverse bulkhead has been used. Individual fibers are spliced and are attached to the bulkhead for support. A disadvantage of this approach is the lack of facilities for the storage of slack in the fibers. In other splicing arrangements, all the optical fibers in a cable are looped within the same retainer or fiber slack is stored on spools. In either case, identification, repair or splice work of individual fibers is extremely difficult without a major rearrangement within the splice closure. This is undesirable because the transmission capability in active fibers can be affected as they are moved.
In another closure of the prior art, there is provided a device for organizing a plurality of individual optical fibers or other similar type conductors or fibers at a slack or splice point. A device having modular construction which is suitable for installation in standard splice closures is provided. The device comprises a plurality of tray-like members each adapted to retain and store at least one fiber. The device provides access to the individual fibers contained in the trays. Each tray may be marked to identify individual fibers therein. The trays are stacked one on top of the other, and each is hinged separately at one end thereof to a carrier, thus allowing them to move relative to one another like bound pages. Each tray-like support has a width which is adequate to provide the minimum bending radius specified for that fiber.
In another prior art optical fiber cable closure, optical fiber transitions with a controlled bend radius are anchored from each cable to a hinged organizer tray. This arrangement provides ready access to in-service optical fibers without the risk of inadvertent bending of the fibers. However, the arrangement of optical fibers in a cable to different trays is somewhat cumbersome to carry out and there appears to be a lack of protection for the fibers in the transition from the cables to the trays. This problem has been solved by the arrangement shown in U.S. Pat. No. 4,927,227, which issued on Apr. 22, 1990 in the names of W. H. Bensel, et al. Therein, a support member includes a support base for supporting an optical fiber breakout and a plurality of splice trays. The breakout allows a user to separate fibers into groups before they are routed to ones of the trays. However, in this last-described closure, there appears to be limited storage capacity and lack of ability to accommodate as many different splicing arrangements as desired. Current thinking would require each tray to store thirty-six splices. In still another closure, each tray includes a plurality of nests formed from a compliant material which allows each nest to receive a plurality of splicing arrangements. However, all of the nests are integral to each tray, although some may not be used immediately, or ever.
What the prior art seemingly lacks is an optical fiber cable closure which provides enhanced storage capability. Enhanced storage capability is interpreted to mean not only enhanced quantity of splice storage spaces over prior art closures, but also the capability to store different kinds of splicing arrangements. Further, in order to control investment, the sought-after closure should be such that a craftsperson has the flexibility of selecting, within a predetermined range, the number of splices which can be accommodated in each tray.